Planets around evolved stars, few key questions that could be tackled with GAIA Lopez Bruno Obs. De la Côte d’Azur, Dpt. Fresnel UMR 6528, BP Nice Cedex4, France. Content : Introduction – few key questions. On the dynamical evolution of planets ? On the formation of planets ? On the possibility for life to develop around Red Giants ? The potential of GAIA.

Planet detections : Discovery and understanding of Earth and Solar System : 5 bodies (Mercury, Venus, Mars, Jupiter, Saturn) at first sight distinguishable from the other stars because changing their place on the sky were discovered very early, Anaxagore ( , Eclipse mechanism), Aristote ( , Earth’s sphericity), Erastothènes ( , mesure of Earth’s diameter), Hipparque (-130, precession of Earth axis), Ptolémée (middle of 2snd century A.D., a model of Solar System), …, Copernic ( , Heliocentrisme), Kepler (1609, elliptical orbits). Discovery and understanding of Extrasolar planetary Systems : has started in with the discovery of planets around pulsars (2 cases apparently confirmed) and has continued with the discovery (since 1994) of giant planets around ‘normal stars’.

On the dynamical evolution of planets ? 1) Mass loss phenomenom and tidal dissipations between planets and the Red Giant star (Rasio et al. 1996) : no remaining planets inside 1 AU around a 1 solar mass star in evolution, Earth’s fate ?, other planets of our Solar System should survive. 2) Resonances in planetary systems like our own should lead to the ejection of planets inside 5 AU.

On the formation of planets ? Since the discovery of planets around pulsar (Wolszczan and Frail 1992) with Msini of 2.8 and 3.4 Earth’s masses and respectively at 0.47 and 0.36 AU, a second generation planet formation is suggested around pulsars that may result from the merging of 2 dwarf stars for instance (Livio et al 1992). More generaly planets may form at different stages of the stellar evolution.

Stingray nebula In about 1000 years …

On the possibility for life to develop around Red Giant stars ? Terrestrial planets, around Reg Giants, in ‘Habitable zone’ offering possibility for life to develop can be discovered by GAIA (Lopez and Schneider 2002 submitted).

Earth’formation Gyrs No trace of fossils Résidus carbonés Gyrs 700 Myrs at most For life occurence Gyrs Oldest fossils known (cyanobacteries) Gyrs Heavy meterotic bombardement during the first Gyr

The potential of GAIA. 1) Red giants are bright and numerous but expected problems for planet detection with GAIA related to surface features linked to convection in the stellar atmosphere. 2) A problem probably relaxed for Sub-Giant stars around which searching For Earth’s like planets in habitable zones transiting long enough for life to Develop is of strong interest. 3) A search of planets around White Dwarfs is very adapted to the potential of GAIA. A SIMBAD/CDS search show that nearly 400 WD of mv less than 15 exist. Type DO, DA, DB.

In a sphere of 20 pc around the Sun (from AQ, Allen) : Jupiter like planets produce well measurable Reflex motion of the star, larger than 10 micro-arcsecs. Hundred of potential sources for such a detection. A 10 Earth’s mass planet produces a reflex motion of the star of the order of 5 micro-arcsecs. Assuming the WD at 10 pc and of 0.6 solar mass and the planet at 1 AU. A few sources.

Interferometry and circumstellar environments

Compagnons stars as a sub-product of this search of planets